1. Field of the Invention
The present invention relates to a semiconductor laser used as a light source in an optical disc device and to a manufacturing method for such semiconductor laser.
2. Description of the Prior Art
Optical disc drives for digital video discs (DVDs) and other such media have been developed in recent years. Of the semiconductor lasers currently available, such devices mainly use AlGaInP-type semiconductor lasers that emit laser light of a short wavelength as their light sources.
In the field of AlGaInP-type semiconductor lasers, much attention has been placed on the favorable characteristics exhibited by RISA (Real refractive Index guided Self-Aligned structure) type lasers. These have been described by Osamu Imafuji et al. on page 1223 of xe2x80x9cElectronics Lettersxe2x80x9d Volume 33 (1997), for example.
FIG. 7 shows a cross-section of a RISA-type laser. The expressions xe2x80x9cabovexe2x80x9d and xe2x80x9cbelowxe2x80x9d in the following explanation refer to the structure when FIG. 7 is in an upright position. The illustrated RISA-type laser has an n-type GaAs substrate 1, on which an n-type GaAs buffer layer 2, an n-type cladding layer 3 made of (AlxGa1-x)yIn1-yP (where x=0.7, y=0.5), an active layer 4, a p-type cladding base layer 5 made of (AlxGa1-x)yIn1-y (where x=0.7, y=0.5), and a current-blocking layer 6 made of AlInP are successively formed in the stated order. Etching is performed next on a stripe-shaped part of the current-blocking layer 6. Above this, a p-type buried cladding layer 7, an ohmic contact layer 8 made of p-type Ga0.5In0.5P, a cap layer 9 made of p-type GaAs are formed in the stated order. A p-type electrode 10 is formed on the cap layer 9, and an n-type electrode 11 is formed on the back of the n-type GaAs substrate 1. Note that the materials cited here are mere examples, so that other combinations of materials may be used.
As shown in FIG. 7, the p-type buried cladding layer 7 that covers the current-blocking layer 6 is buried in a groove formed in the center of the construction. This produces a current flow concentrating effect whereby the flow of current between the top and bottom of the p-type buried cladding layer 7 is narrowed. Light is confined within the n-type cladding layer 3, the p-type cladding base layer 5, and the p-type buried cladding layer 7. The current flow concentrating and light-confining effects of this construction ideally mean that laser light can be produced using a relatively low operating current.
FIG. 8 shows the manufacturing process for the above type of laser. Each layer from the n-type GaAs buffer layer 2 to the cap layer 9 is successively formed using metalorganic vapor phase epitaxy (MOVPE). In more detail, each layer up to the current-blocking layer 6 is successively formed on the n-type GaAs substrate 1 (process 1). Once the current-blocking layer 6 has been provided, a stripe is formed by etching a central part of the current-blocking layer 6. Before the p-type buried cladding layer 7 is formed, impurities (which are mainly composed of the etching solution that remains after the etching process) need to be removed from the surface of the multilayer structure formed of the n-type GaAs substrate 1 to the current-blocking layer 6. These impurities are removed by a thermal cleaning process where the multilayer structure is heated at a high temperature (generally 700xc2x0 C. or higher) that is near the crystal growth temperature of the layers (process 2). To prevent phosphorous from being vaporized from the surface of the multilayer structure, a phosphorous compound such as phosphine (PH3) or the like is supplied during this process. In this way, the cleaning is performed in the presence of a phosphorous compound. The remaining layers are thereafter formed using MOVPE, and the manufacturing procedure ends with the formation of the electrodes (process 3).
To improve the performance of optical disc devices in which the above laser is used, however, it is desirable to further improve the laser characteristics, such as by lowering the laser threshold current (see Optical Device Dictionary, page 8).
It is a first object of the present invention to provide a durable semiconductor laser with improved laser characteristics, such as a lowered threshold current.
It is a second object of the present invention to provide a manufacturing method for efficiently producing a durable semiconductor laser with improved laser characteristics, such as a lowered threshold current.
In order to achieve the stated objects, the inventors studied the manufacturing method of the semiconductor lasers described in the prior art, and tried to find points that could be improved. As a result, the inventors found that when thermal cleaning is performed as part of the manufacturing method of the above semiconductor laser, the etching proceeds in the horizontal direction at the joins between the current block layer and the p-type cladding base layer, creating concaves in the current-blocking layer. If the current-blocking layer is embedded and reconstructed in this state having concaves, crystal growth will not proceed in certain areas and, as shown in FIG. 7, hollows 12 will be left in the construction. While these hollows 12 may be small, they are formed along the interface between the p-type cladding base layer 5 and the current-blocking layer 6 and cause waveguide loss in the semiconductor laser. This worsens the semiconductor laser characteristics (such as by raising the threshold current) and so has made it impossible to realize an ideal semiconductor laser.
The inventors next focused on the horizontal progression of the etching that occurs at the interface between the p-type cladding base layer 5 and the current-blocking layer 6 during the thermal cleaning process in terms of the concentration of carriers in the current-blocking layer 6. As a result, the inventors discovered that the extent to which etching progresses in the horizontal direction at the interface between the p-type cladding base layer 5 and the current-blocking layer 6 has a high correlation with the concentration of carriers in the current-blocking layer 6. This discovery led to the conception of the present invention.
In order to achieve the stated first object, the present invention is a semiconductor laser, including: an n-type cladding layer that has n-type conductivity; an active layer formed on top of the n-type cladding layer; a p-type cladding base layer that is formed on top of the active layer and has p-type conductivity; a current-blocking layer that is formed on specified parts of an upper surface of the p-type cladding base layer and substantially has n-type conductivity; and a p-type buried cladding layer that has p-type conductivity and is formed so as to cover the current-blocking layer and contact remaining parts of the upper surface of the p-type cladding base layer, wherein the current-blocking layer has at least two regions having different concentrations (hereafter xe2x80x9cN1xe2x80x9d and xe2x80x9cN2xe2x80x9d where N1 less than N2) of n-type carriers, a region adjacent to an interface between the p-type cladding base layer and the p-type buried cladding layer having the N1 concentration of n-type carriers and a part or all of a remaining region of the current-blocking layer region having the N2 concentration.
Note that the term xe2x80x9csubstantiallyxe2x80x9d used in the description of the current-blocking layer means that while one or more parts of the current-blocking layer 6 may not have a current blocking action, such action will be achieved by some other part of the current-blocking layer. This is also the case for the other independent claims.
In the semiconductor laser of the stated construction, the concentration of carriers in the current-blocking layer near the joins between the p-type cladding base layer and the p-type buried cladding layer is lower than the concentration of carriers in some or all of a remaining part of the current-blocking layer. Such other part ensures that the current narrowing effect of the current-blocking layer is maintained, even though etching is suppressed at the joins between the p-type cladding base layer and the p-type buried cladding layer during the thermal cleaning process. As a result, the hollows produced at such joins are smaller than the hollows in conventional semiconductor lasers, which lowers the threshold current and raises the slope efficiency, thereby improving the laser characteristics. Since this improved performance enables a high output to be achieved with a lower current, reliability is also improved.
It is not clear how the above reduction in the size of the hollows was achieved, but it is believed that this results from the following action.
The region of the current-blocking layer near the bottom surface that contacts the upper surface of the p-type cladding base layer has a relatively high concentration of n-type carriers. During thermal cleaning, the carriers are subjected to heat dispersion and PN junctions occur between the moved carriers. As a result, a depletion layer is formed at the interface between the p-type cladding base layer and the current-blocking layer. Charge builds up in this depletion layer, and since the layer is exposed to the gas atmosphere (such as phosphine), an electrochemical reaction occurs and the current-blocking layer is etched in the horizontal direction.
In the current-blocking layer of the present invention, the concentration of carriers in the current-blocking layer near the interface with the p-type cladding base layer and the p-type buried cladding layer is lower than part or all or the remaining region in the current-blocking layer. As a result, the depletion layer is reduced and fewer electrochemical reactions occur with the surrounding gas atmosphere during the thermal cleaning, while the current blocking action is maintained by the parts of the current-blocking layer with the higher concentration of carriers. Note that when the concentration of carriers is constant within the entire current-blocking layer, the setting of a lower concentration of carriers will similarly result in fewer electrochemical reactions during the thermal cleaning, though such current-blocking layer will not effectively narrow the flow of current in the semiconductor laser.
FIG. 2 shows the results of an investigation into the relationship between the concentration of n-type carriers in the part of the current-blocking layer that is near the upper surface of the p-type cladding base layer and the extent to which etching occurs. The black circles in FIG. 2 indicate measurements of etching speed, while the solid line shows the optimum curve. This experiment was performed for the first embodiment of the present invention, where the etching speed is measured while the concentration of n-type carriers in the part of the current-blocking layer that is near the upper surface of the p-type cladding base layer varies within the range given as 0 to 1*1018 cmxe2x88x923. Note that the values in FIG. 2 expressing the concentration of carriers are concentrations in the materials used for forming the various layers and were measured according to a C-V method.
As shown in FIG. 2, the lower the concentration of n-type carriers in part of the current-blocking layer near the upper surface of the p-type cladding base layer, the slower the etching speed, with the etching speed being virtually zero when the concentration of carriers is 1*1017 cmxe2x88x923 or below.
These results show that a reduction in the concentration of carriers in a part of the current-blocking layer near the interface with the p-type cladding base layer and the p-type buried cladding layer is effective in reducing the size of the hollows. Since the etching speed is virtually zero when the concentration of carriers is 1*1017 cm3 or below in the part of the current-blocking layer near the p-type cladding base layer, setting the concentration of carriers in this range almost eradicates the hollows altogether from the structure. Note that this concentration of carriers is not the concentration in the finally produced semiconductor laser, but instead the value before thermal cleaning, although this will be substantially the same as the concentration in the finished product.
Here, the current-blocking layer may includes a first layer that contacts the p-type cladding base layer and a second layer that is provided on top of the first layer, a concentration of n-type carriers in the first layer being N1 and a concentration of n-type carriers in the second layer being N2.
With this structure, the first of the two layers has a low concentration of carriers, which suppresses etching during the thermal cleaning. The second of the two layers has a high concentration of carriers and so blocks the flow of current.
Here, the first layer may have a different composition to the second layer.
In a semiconductor laser with the above construction, the dispersion of carriers during the thermal cleaning is suppressed by the hetro interface between the first and second layers. The formation of a depletion layer at the interface with the p-type cladding base layer due to the effects of dispersed carriers is therefore suppressed, so that etching can be prevented more effectively.
Here, one of the first layer and the second layer may be composed of a plurality of sublayers that have at least two different compositions in a semiconductor laser with the above construction, the dispersion of carriers during the thermal cleaning is suppressed by the hetro interfaces between the sublayers in the first or second layer. The formation of a depletion layer at the interface with the p-type cladding base layer due to the effects of dispersed carriers is therefore suppressed, so that etching can be prevented more effectively.
In claim 4, the statement that the sublayers have xe2x80x9cdifferent compositionsxe2x80x9d does not simply mean that there are different concentrations of impurities, but instead can also mean that there are different metals in the respective compositions or that the compositions include different proportions of the same metals.
Here, the second layer may be co-doped with a p2 concentration of p-type carriers and an n2 (where n2 greater than p2) concentration of n-type carriers, and n2 and p2 may be set so that n2xe2x88x92p2=N2.
In a semiconductor laser with the stated construction, the dispersion of carriers during the thermal cleaning is prevented within the second layer. The formation of a depletion layer at the interface with the p-type cladding base layer due to the effects of dispersed carriers is therefore suppressed, so that etching can be prevented more effectively.
The first object of the present invention can also be achieved by a semiconductor laser, including: an n-type cladding layer that has n-type conductivity; an active layer formed on top of the n-type cladding layer; a p-type cladding base layer that is formed on top of the active layer and has p-type conductivity; a current-blocking layer that is formed on specified parts of an upper surface of the p-type cladding base layer and substantially has n-type conductivity; and a p-type buried cladding layer that has p-type conductivity and is formed so as to cover the current-blocking layer and contact remaining parts of the upper surface of the p-type cladding base layer, the current-blocking layer having a region with p-type conductivity adjacent to the interface between the p-type cladding base layer and the p-type buried cladding layer and another region with n-type conductivity.
In a semiconductor laser with the stated construction, the part of the current-blocking layer near the interface with the p-type cladding base layer and the p-type buried cladding layer has p-type conductivity. As a result, a depletion layer is formed at a position within the structure that is shifted upward from the interface between the p-type cladding base layer and the current-blocking layer. During the thermal cleaning, this shifted depletion layer is positioned at some distance from the position where the gas atmosphere tends to linger, which reduces the frequency with which the depletion layer comes into contact with the gas atmosphere. This reduces the extent to which liquid-phase etching occurs, while the region of the current-blocking layer with n-type conductivity maintains the current-narrowing effect.
The first object may also be achieved by a semiconductor laser including: an n-type cladding layer that has n-type conductivity; an active layer formed on top of the n-type cladding layer; a p-type cladding base layer that is formed on top of the active layer and has p-type conductivity; an interjacent layer that has p-type conductivity and is formed on specified parts of an upper surface of the p-type cladding base layer and contacts the p-type cladding base layer; a current-blocking layer that is formed on the interjacent layer and has n-type conductivity; and a p-type buried cladding layer that has p-type conductivity and is formed so as to cover the current-blocking layer and contact remaining parts of the upper surface of the p-type cladding base layer, the interjacent layer being positioned between the current-blocking layer and the p-type cladding base layer so that a lower surface of the current-blocking layer is separated from an upper surface of the p-type cladding base layer.
Since an interjacent layer with p-type conductivity is present between the p-type cladding base layer and the current-blocking layer, a depletion layer is formed at a position within the structure that is shifted upward to an interface between the interjacent layer and the current-blocking layer. During the thermal cleaning, this shifted depletion layer is positioned at some distance from the position where the gas atmosphere tends to linger, which reduces the frequency with which the depletion layer comes into contact with the gas atmosphere. This reduces the extent to which liquid-phase etching occurs.
The second object of the present invention can be realized by a semiconductor laser manufacturing method, including: a first process for successively forming an n-type cladding layer having n-type conductivity, an active layer, and a p-type cladding base layer having p-type conductivity on top of one another, before forming a current-blocking layer, which substantially has n-type conductivity, on specified parts of an upper surface of the p-type cladding base layer; a second process for performing thermal cleaning in a presence of a specified gas after the first process has finished; a third process for forming, after the second process has finished, a p-type buried cladding layer, which has p-type conductivity, so as to cover the current-blocking layer and contact remaining parts of the upper surface of the p-type cladding base layer, the first process including: a first subprocess for forming a region of the current-blocking layer that is adjacent to the interface between the p-type cladding base layer and the p-type buried cladding layer with a concentration (hereafter, xe2x80x9cN1xe2x80x9d) of n-type carriers; and a second subprocess for forming another region in at least part of the current-blocking layer with a concentration (hereafter, xe2x80x9cN2xe2x80x9d) of n-type carriers, where N1 less than N2.
In a semiconductor laser produced by the stated manufacturing method, the concentration of carriers in the current-blocking layer near the joins between the p-type cladding base layer and the p-type buried cladding layer is lower than the concentration of carriers in some or all of a remaining part of the current-blocking layer. Such other part ensures that the current narrowing effect of the current-blocking layer is maintained, even though etching is suppressed at the joins between the p-type cladding base layer and the p-type buried cladding layer during the thermal cleaning process. As a result, the hollows produced at such joins are smaller than the hollows in conventional semiconductor lasers, which lowers the threshold current and raises the slope efficiency, thereby improving the laser characteristics. Since this improved performance enables a high output to be achieved with a lower current, reliability is also improved.
Here, the first process may produce the current-blocking layer by forming a first layer that contacts the p-type cladding base layer and a second layer on top of the first layer, a concentration of n-type carriers being N1 in the first layer and N2 in the second layer.
The stated manufacturing method produces a structure where the first of the two layers has a low concentration of carriers, which suppresses etching during the thermal cleaning. The second of the two layers has a high concentration of carriers and so blocks the flow of current.
It is preferable for the values 0cmxe2x88x923xe2x89xa6N1xe2x89xa61017 cmxe2x88x923 and N2 greater than 1017cm xe2x88x923 to be used.
Here, the first process may form the first layer from a different composition of materials to the second layer.
In a semiconductor laser produced by the stated manufacturing method, the dispersion of carriers during the thermal cleaning is suppressed by the hetro interface between the first and second layers. The formation of a depletion layer at the interface with the p-type cladding base layer due to the effects of dispersed carriers is therefore suppressed, so that etching can be prevented more effectively.
Here, the first process may produce one of the first layer and the second layer by forming sublayers from at least two different compositions of materials.
In a semiconductor laser produced by the stated manufacturing method, the dispersion of carriers during the thermal cleaning is suppressed by the hetro interfaces between the sublayers in the first or second layer. The formation of a depletion layer at the interface with the p-type cladding base layer due to the effects of dispersed carriers is therefore suppressed, so that etching can be prevented more effectively.
Here, the first process may co-dope the second layer with a p2 concentration of p-type carriers and an n2 (where n2 greater than p2) concentration of n-type carriers, where N2=(n2xe2x88x92p2)
In a semiconductor laser produced by the stated manufacturing method, the dispersion of carriers during the thermal cleaning is prevented within the second layer. The formation of a depletion layer at the interface with the p-type cladding base layer due to the effects of dispersed carriers is therefore suppressed, so that etching can be prevented more effectively.
The second object of the present invention can also be realized by a semiconductor laser manufacturing method, including: a first process for successively forming an n-type cladding layer having n-type conductivity, an active layer, and a p-type cladding base layer having p-type conductivity on top of one another, before forming a current-blocking layer, which substantially has n-type conductivity, on specified parts of an upper surface of the p-type cladding base layer; a second process for performing thermal cleaning in a presence of a specified gas after the first process has finished; a third process for forming, after the second process has finished, a p-type buried cladding layer, which has p-type conductivity, so as to cover the current-blocking layer and contact remaining parts of the upper surface of the p-type cladding base layer, the first process forming the current-blocking layer so as to include a region with n-type conductivity and a region with p-type conductivity, the first process including: a first subprocess for forming a region with p-type conductivity adjacent to an interface between the p-type cladding base layer and the p-type buried cladding layer; and a second subprocess for forming a region with n-type conductivity in at least part of a remainder of the current-blocking layer.
In a semiconductor laser produced by the stated manufacturing method, the part of the current-blocking layer near the interface with the p-type cladding base layer and the p-type buried cladding layer has p-type conductivity. As a result, a depletion layer is formed at a position within the structure that is shifted upward from the interface between the p-type cladding base layer and the current-blocking layer. During the thermal cleaning, this shifted depletion layer is positioned at some distance from the position with the gas atmosphere tends to linger, which reduces the frequency with which the depletion layer comes into contact with the gas atmosphere. This reduces the extent to which liquid-phase etching occurs, while the region of the current-blocking layer with n-type conductivity maintains the current-narrowing effect.
The second object of the present invention can also be realized by a semiconductor laser manufacturing method, including: a first process for successively forming an n-type cladding layer having n-type conductivity, an active layer, a p-type cladding base layer having p-type conductivity, and an interjacent layer that has p-type conductivity and contacts the first p-type cladding base layer on top of one another, before forming a current-blocking layer, which substantially has n-type conductivity, on an upper surface of the interjacent layer; a second process for performing thermal cleaning in a presence of a specified gas after the first process has finished; a third process for forming, after the second process has finished, a p-type buried cladding layer, which has p-type conductivity, so as to cover the current-blocking layer and contact remaining parts of the upper surface of the p-type cladding base layer, the interjacent layer being formed between the current blocking layer and the p-type cladding base layer so that a lower surface of the current-blocking layer is separated from an upper surface of the p-type cladding base layer.
In a semiconductor laser produced by the stated manufacturing method, an interjacent layer with p-type conductivity is present between the p-type cladding base layer and the current-blocking layer. As a result, a depletion layer is formed at a position within the structure that is shifted upward to an interface between the interjacent layer and the current-blocking layer. During the thermal cleaning, this shifted depletion layer is positioned at some distance from the position with the gas atmosphere tends to linger, which reduces the frequency with which the depletion layer comes into contact with the gas atmosphere. This reduces the extent to which liquid-phase etching occurs.